[biscuitville]

Can anyone provide some "rules of thumb" with respect to jetting changes as a function of temperature? I was specifically wondering how the changes made to higher performance engines compared to changes one would make to milder performing street engines (Holley 4150 style carbs in mind).
Thanks

[curtis73]

The carb compensates for temperature by itself. If it gets colder, the air entering the carb is denser, which means it sucks more fuel with it. With extremely cold air or a cold engine your choke compensates for any additional fuel you need in the ratio.

If its jetted properly for 90 degrees, it should work fine at 30 degrees.

[Rustydawg]

Somewhere I have a photocopied chart from the Ski-Doo racing manual - it lists what jetting to use by altitude and temperature. The jetting changes are significant for a 10 degrees C change. I would think the same amount of change would apply to a Holley 4150, you'd have to do some math.

The info applies to Mikuni VM carbs, but the math should be applicable to Holleys if you can calculate jet area.

I'll post a note if I can find it...

[Rustydawg]

I found the sled jetting info, it's not as detailed a 'chart' as I thought, but still good reference. For a given temperature, they jet down two sizes for every 2000 ft increase in altitude, so one jet size for every 1000 ft.

Similarly, they suggest to jet up one size for every 20* F drop in temp.

So a 20 degree cooler day is similar to going 1000 ft down in altitude.

David Vizard dwells on cool air in one of his books, his rule of thumb is a 1% increase in density for every 6* F of temperature reduction.

If a Holley jet is rated in flow rate (as I seem to recall) then you should be able to jet up or down from the 1% per 6 degree rule.

[Stroke]

Here in MI the rule of thumb has always been that if you run a carb you are going to mess with it at some point, especially if you drive it year round. Sure, you can get it dialed in 'good enough' to leave alone, but what fun is that?

[curtis73]

I have operated all of my Qjets at temperatures ranging from -20F to 115F without changing a single thing. Altitudes over about 3000 would start to lower the idle, but if you have to re-jet for temp changes, something isn't right.

I can understand when you are trying to get every last HP you might try different jets, but if you stick an A/F meter on my Qjets, I guarantee you'll get almost identical readings regardless of air temperature.

[Rustydawg]

Biscuitville, here's a 'rule of thumb' for you, from "Holley Carburetors & Manifolds" by Mike Urich and Bill Fisher. You really got my interest tweaked on this one 'cause I'm contemplating buying a 0-4777 and I've been reading up on them and have this book handy.

In this book their rule of thumb is that density drops 1% for every 11* F increase. David Vizard claims 1% for every 6* F, I can't explain the discrepancy. I believe it's a simple gas law from high school physics but I don't remember that stuff.

They go on to say "main jet corrections of approximately one main jet size smaller for every 40* F ambient temperature increase can be made to keep mixture ratio correct" There's your rule of thumb.

Interestingly, they note that Holley engineers figure there is approximately 4% fuel-flow change between jet sizes - so the math checks.